A great how-to and case study article on a protyping technique called sketchboarding. And an overview of the different purposes and benefits of sketching.
Even if it's not something you can try, the article does a good job of illustrating the benefits of early communication, trying out ideas and not getting bogged down in technology.
Using Sketchboards to design great User Interfaces quickly - article by James Downes for Box UK
Jared Spool has also written a nice piece on sketching - outlining different reasons for doing it, with examples and benefits for each case.
One of the main things here is that you shouldn't get hung up on your sketching ability. Rough and ropey is fine - so long as it communicates an idea and facilitates understanding or consensus.
Why We Sketch - article by Jared Spool